It's the same membrane all eggs have under the shell. The only thing missing with ovoviviparous snakes, is the shell. The mother has temporary fine blood vessels that are attached to this membrane during gestation. This provides gas exchange for the babies. She does their breathing for them. The membrane also keeps out pathogens, fungi, etc. Inside the membrane, is a completely sterile environment while still allowing gas exchange. Although I have no proof, I think that when fully developed babies are born dead, I think something goes wrong with the timing. Mothers blood vessels obviously must detach from the membrane in order for the eggs to move down and out. If this happens too soon before the eggs come out, the babies would suffocate.